Process Line Sizing by Velocity
Finds the inside diameter a pipe needs to carry a flow at a chosen velocity, and checks that velocity against the API RP 14E erosional velocity [1]. Process and piping engineers use it for a first line size, before choosing a standard pipe and checking the pressure drop.
Equations
Q = ṁ/ρ D = √(4Q/(πV))
ve = C/√ρ
Q is the volumetric flow, ṁ the mass flow and ρ the fluid density. V is the target velocity and D the inside diameter that gives exactly that velocity. ve is the erosional velocity, the speed above which API RP 14E expects the flow to erode the pipe wall [1]. C is an empirical constant set by the service.
API RP 14E gives C in US units, with ve in ft/s and ρ in lb/ft³. The page converts it exactly to SI, with ve in m/s and ρ in kg/m³: C = 100 in US units is 121.99 in SI.
Inputs
- Choose Flow Basis Mass flow + density when the flow comes from a heat and material balance. Choosing a Fluid fills in its density [4], [6], [7].
- Pick the target velocity from the usual ranges for the service [2], [3]. Typical values are 1 to 3 m/s for liquid lines, below about 1 m/s for pump suctions and 10 to 30 m/s for gases.
- The API RP 14E Service list gives C = 100 for continuous service and 125 for intermittent service, for solids-free fluids. Higher values (150 to 250) apply only where corrosion is absent or controlled.
Limits and common mistakes
- D is an exact inside diameter, not a pipe size. Choose the next standard size up and recheck its velocity. The inside diameter of a standard pipe depends on its schedule.
- The erosional limit was written for oil and gas production lines, where it is most often applied to two-phase flow. Use the mixture density for two-phase lines. It does not cover sand, which needs a lower limit.
- Velocity alone does not size a line. Check the pressure drop, and the NPSH for pump suctions.
- For gases, use the density at the flowing pressure and temperature, not at standard conditions.
Worked example
A seawater cooling line must carry 180,000 kg/h at no more than 2.5 m/s, in continuous service. What inside diameter does it need, and is it safe from erosion?
- Choose Flow Basis Mass flow + density and enter Mass Flow 180000 kg/h.
- Choose Fluid Seawater, 20°C. The density fills in as 1,025 kg/m³.
- Enter Target Velocity 2.5 m/s.
- Leave API RP 14E Service on Solids-free, continuous service — C = 100, and press Solve. The Solution shows:
Volumetric flow: 175.61 m³/h
Required inside diameter: 157.619 mm
Velocity at that diameter: 2.5 m/s
API RP 14E erosional velocity ve: 3.81034 m/s
Target velocity / erosional velocity: 0.65611
Assessment: Below erosional limit
Erosional constant C (US units: ft/s, lb/ft³): 100
Erosional constant C (SI units: m/s, kg/m³): 121.99
Check by hand: Q = 180,000/1,025 = 175.6 m³/h, or 0.04878 m³/s, so D = √(4 × 0.04878/(π × 2.5)) = 0.1576 m. In US units, ρ = 63.99 lb/ft³ and ve = 100/√63.99 = 12.50 ft/s, or 3.81 m/s. A 6-inch Schedule 40 pipe (154.1 mm inside) would run at 2.62 m/s, over the target. An 8-inch Schedule 40 pipe (202.7 mm inside) runs at 1.51 m/s.